Chapter 18 Change Over Time. Plate Tectonics Earth’s crust has changed throughout geologic time. Earth’s crust and the solid part of the mantle form the

Chapter 18

Change Over Time

Plate Tectonics

• Earth’s crust has changed throughout geologic time. Earth’s crust and the solid part of the mantle form the lithosphere. The lithosphere is where volcanoes and mountains form. The lithosphere can break because it is fragile. It breaks up into thick, moving slabs of rock called tectonic plates. Tectonic plates fit together like one big jigsaw puzzle. The study of how the plates move is called plate tectonics.

Plate tectonics continued

• Tectonic plates move by convection currents that form deep in Earth’s interior. The currents bring heated material upwards while pulling cooler material down towards the interior of the Earth. The heating and cooling motions move the overlying crust along. The rate of movement is about 1-9 cm per year.

Alfred Wegener

• Early scientists studying the configuration of the Earth noted how the continents appeared to look like a puzzle. The pieces are now separate but could fit together. In 1915, a scientist named Alfred Wegener introduced the concept of moving continents. He also came up with a theory that all the Earth’s land masses were once part of a supercontinent, called Pangea.

Pangea

• Pangea broke up over millions of years and became the Earth that we now live on. This theory is called continental drift theory. Later scientists confirmed Wegner’s idea by linking fossils and rock units between land masses now separated by great distances. Similar fossils and rock units could only occur if the land masses were once joined together. Seismic data from around the world was used to show the presence of fractures in Earth’s crust. It showed where earthquakes and volcanic activity were occurring. Today there are 17 major tectonic plates on Earth.

Environments Change Over Time

• The movement of tectonic plates has major consequences for the plants and animals that live on Earth. When a plate moves, the plants and animals that live on the plates also move. Environmental conditions (like sunlight, temp, and rainfall) change in response to the tectonic plate movement. A plate located near the equator would have lots more sunlight and be warmer than a plate located at one of the Earth’s poles.

Environments Change Over Time Continued

• When land masses join, diversity (different species) decreases. In a large space, organisms can easily find suitable habitats. When land masses break apart, diversity increases. Organisms on a small landmass must make do with the current habitat. They cannot move to a new space and must survive under challenging, stressful conditions with limited resources. Smaller landmasses may have fewer competitors or predators.

Environments Change Over Time

• Most natural environmental change is slow. The formation of the Isthmus of Panama took 12 million years to complete. This allows organisms the opportunity to change to fit their new environment. The gradual changes of an organism, from one form to another over many generations, form the process of evolution. The first person to propose the idea of evolution was Charles Darwin.

Charles Darwin

• Charles Darwin traveled the world observing living animals and fossils on different continents. These observations and many years of study allowed him to propose a valid mechanism explaining how organisms change from one form to another.

How Evolution Happens

• Natural Selection– Darwin repeatedly observed how environmental

pressures can change the way an organism naturally interacts with its environment. He developed the theory of evolution from his studies. Evolution is the modification of organisms over many generations. It is a result of natural selection. The process of natural selection states that the organisms best suited to the environment will be most likely to survive and produce offspring.


• Natural Selection– Darwin realized that organisms must compete for

resources like food, water and space. Those organisms that perform the best are most likely to get food, water and space and will live longer lives and produce more offspring. Scientists call the ability of an organism to live, survive and reproduce in a particular environment fitness. Not all individual animals within a population have the same fitness.


• Natural Selection-– This variation produced through sexual reproduction

makes some organisms more fit than others. These different characteristics determine the chances of an organisms survival. This is where the idea of survival of the fittest comes from. Organisms that are the fittest will live the longest and produce the most offspring. Favorable traits that help the species survive are called adaptations. An adaptation can be a physical structure or a behavior. Adaptations are inherited by organisms, and they help the organism to survive.


• Competition-– According to Darwin, competition is one main

driving force behind evolution. The interactions between different species and between individuals of the same species cause some organisms to survive while others perish. Resources like food, water and sunlight are limited this causes organisms to compete to obtain the resources.

How Evolution Happens- Biological Evidence

• Environmental Impact On Organisms-Ostriches Emus


• Ostriches and emus are 2 similar looking birds found on 2 different continents. One lives in Africa and the other lives in Australia. DNA evidence confirms these 2 birds are related. They are flightless birds that came from a prehistoric continent called Gondwanaland. This was a massive supercontinent that formed about 550 million years ago and it included most of the continents that now make up the southern hemisphere.

How Evolution Happens- Biological

• When it broke up about 200 million years ago, the animals living on these land masses traveled to their current locations. Each bird population found itself living in a new environment. They now had new environmental pressures to deal with. These modern birds are descendants of a common ancestor that once lived on Gondwanaland. These birds show common genetic, behavioral, and physical characteristics.


• The ostrich, emu, rhea, cassowary and kiwi are all members of the same order of birds. They share some similarities and have some differences.– Ostrich and cassowary are boldly colored– Emu and rhea are camouflaged– Cassowary is smaller because it lives on an island– Ostrich is taller– Ostrich has lost a toe and emu has not– Cassowary’s feathers are more like hair to protect the

bird from plants

How Evolution Occurs-Biological Evidence

• The individual physical traits are not modified by the bird. Rather, the birds that were born with the trait that is favored by the current environmental pressures survive and pass that trait on to their offspring. This ensures that over time, the expression of the favored trait becomes more pronounced, and birds lacking favored traits die off.

How Evolution Occurs- Biological Evidence

• Often when populations are geographically separated, like the ostriches and emus, 2 new populations are formed. The 2 new populations are physically separated and can no longer breed with the “lost” members. This means that some traits can quickly predominate in a population because of the closed breeding system. This increases the rate at which species change.


• Theories of evolution can also be supported by anatomical similarities or the similarities in the structure of living things.– Homologous structures: are parts/organs that are

similar in shape and have developed from a common ancestor but do not perform the same function. For example, human arm and wing of a bird and flipper of a whale all contain the same bones


• Vestigial organs: structures that are no longer used or have greatly decreased in importance. For example, a whale and some snakes have a pelvis and femur, which are structures necessary for walking. But they do not walk. Their presence suggests a common ancestor.

• Analogous structures: are similar in function but they come from different ancestors. The porpoise (mammal) and the shark (fish) both have long smooth bodies and fins which closely match in appearance and function but they did not come from the same ancestor.

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Chapter 18 Change Over Time. Plate Tectonics Earth’s crust has changed throughout geologic time. Earth’s crust and the solid part of the mantle form the